This disclosure generally relates to drill string flow control valves and more particularly, drill string flow control valves for prevention of u-tubing of fluid flow in drill strings and well drilling systems.
Managed Pressure Drilling (MPD) and Dual Gradient Drilling are oilfield drilling techniques that often utilize a higher density of drilling mud inside the drill string and a lower density return mud path on the outside of the drill string.
In Dual Gradient Drilling, an undesirable condition called “u-tubing” can result when the mud pumps for a drilling system are stopped. Mud pumps are commonly used to deliver drilling mud into the drill string and to extract return mud from the wellbore and a return riser (or risers). In a typical u-tubing scenario, fluid flow inside a drill string may continue to flow, even after the mud pumps have been powered down, until the pressure inside the drill string is balanced with the pressure outside the drill string, e.g., in the wellbore and/or a return riser (or risers). This problem is exacerbated in those situations where a heavier density fluid precedes a lighter density fluid in a drill string. In such a scenario, the heavier density fluid, by its own weight, can cause continued flow in the drill string even after the mud pumps have shut off. This u-tubing phenomenon, can result in undesirable well kicks, which can cause damage to a drilling system. For this reason, it is desirable that when mud pumps in a drilling system are turned off, the forward fluid flow be discontinued quickly.
Drill string flow control valves or flow stop valves are sometimes used to control flow in a downhole tubular, which may be, or form part of, a drill string. Some drill string flow control valves utilize the pressure differential between certain pressure ports positioned along the primary flow path of the valve to apply pressure to a valve sleeve within a valve housing to cause actuation of the valve sleeve. Movement of the valve sleeve, in turn, opens or closes the main drilling fluid flow ports within the valve. In prior art valves, at least two know drawbacks exist. First, to open the sleeve, significant forces maintaining the sleeve in a closed position must initially be overcome. Second, a rapid opening of the sleeve can cause a significant pressure drop in the valve. Thus, in some flow control valves, in order to overcome the significant forces maintaining the sleeve in a closed position, a solid piston is used to slowly initiate movement of the valve sleeve. As the valve sleeve of a prior art flow control valve is initially urged into the open position by the solid piston, flow through the main flow ports of the flow control valve begins. With respect to pressure drops within the valve, those skilled in the art will understand that because the main flow ports are relatively large, as they begin to open, just a small amount of movement of the valve sleeve can cause a drop in pressure as the ports open. For this reason, the solid piston described above is also desirable because it permits the valve sleeve to be opened slowly, thereby minimizing pressure drop. However, by slowly opening the main flow ports utilizing such a solid piston, the fluid flow passing through the ports is maintained at a high pressure, thereby causing potential washout of the flow ports, i.e., the high velocity of the fluid passing through the partially-open main flow ports will corrode or wash away the steel from which such flow control valves and main flow ports are typically fabricated.